1. Field of the Invention
The present invention relates to an apparatus and a method for measuring a refractive index distribution of an object such as an optical element.
2. Description of the Related Art
Optical elements such as lenses used in optical apparatuses such as digital cameras and laser beam printers require a high refractive index. Moreover, using a mold forming technique makes it easy to form a complex shape such as an aspheric surface from an optical glass and a plastic material having a high refractive index.
However, the molding forming technique may cause unevenness of the refractive index in the optical element depending on forming conditions. Such unevenness of the internal refractive index may provide significant influences on optical characteristics of the optical element, which makes it impossible to achieve desired optical characteristics. Thus, it is necessary to accurately measure optical homogeneity inside the optical element.
Japanese Patent Laid-Open No. 2010-151578 discloses a method of acquiring a refractive index distribution of an object by measuring transmitted wavefronts of the object in states where the object is soaked in two media having refractive indices different from each other and from that of the object. This measuring method enables highly accurate measurement of the internal refractive index distribution of the object when the object has a high refractive index, without using a medium having a refractive index approximately equal to that of the object.
The measuring method disclosed in Japanese Patent Laid-Open No. 2010-151578 requires accurate measurement of placement of the object such as tilt or decentering. However, when the object has a complex shape like an optical element such as a lens, it is difficult to accurately measure the placement of the object, which makes it difficult to accurately calculate the refractive index distribution of the object.